Though the occasional user of photo-lithographic plates may still process an exposed plate manually, the large-scale commercial utilization of offset plates has made automatic processing of these plates an economic necessity. Moreover, it is well-known that chemicals utilized in the processing of offset plates may be notably injurious to humans and due care must be exercised to prevent harm resulting from exposure to the chemicals.
Manual processing of an exposed offset plate comprises pouring a sufficient quantity of developer on the plate and working the developer over the area of the plate until a dense, even image appears. If the developer expends itself before the plate is fully developed, more developer is added. If the developer becomes tacky, water is added to the sponge to restore a desirable consistency to the developer. After the plate is developed, it is flushed with water and excess water is then removed, as for example, with a squeegee. This method of manual development is essentially the same irrespective as to whether the process is an additive or subtractive process. The developed plate may thereafter be "gummed" if desired, to preserve the plate.
The continuation of manual processing of plates with currently available chemicals is being legislatively discouraged by requirements of government agencies relative to the use of dangerous chemicals, which requirements place severe limitations as to the acceptability of processing plates manually, that is, without breathing some of the vapors or permitting accidental contact of the chemicals.
Again, chemicals used in processing offset plates are notably difficult to dispose of because of their toxic effect on living matter in the environment.
Stricter enforcement of laws with respect to discarding chemicals into conventional sewer systems places a high value on a recirculating and holding tank for chemicals. Thus it is highly desirable to utilize an integrated or self-contained machine in which the chemicals may be recirculated to depletion, prior to being disposed of in a suitable manner.
The foregoing combination of circumstances dictates the use of an automatic self-contained basic-unit processor even for the economically handicapped user of offset plates.
The foregoing remarks are directed equally to additive type and subtractive type developing machines such as are conventionally used at present in the printing industry. They are more particularly directed to subtractive type developing machines. Machines specifically adapted to process exposed-lithographic plates automatically are disclosed in U.S. Pat. Nos. 3,608,464; 3,593,641; 3,589,261; 3,562,834; 3,448,720; and others.
Unlike prior art machines, the basic-unit processor of this invention performs only a single process function at any one time. This apparent limitation affords the unit exceptional versatility as a basic-unit capable of being used as a developer or a coater; as a developer it may be used for subtractive or additive developers; as a coater it may be used for coating a metal plate with a photosensitive material, or for coating a developed offset plate with "gum."
The coating of aluminum plates with a wipe-on diazo coating has been a relatively recent development. Initially, and in a few instances where only a very few plates are to be coated, the diazo coating fluid is poured onto a plate and wiped more or less evenly over the surface with a clean, lintless cloth. The plate is thereafter dried. This manual coating of aluminum plates has been superceded by an automatic coater wherein a pair of coupled drive rolls is disposed above the surface of the coating fluid, with the lower roll partially submerged in the coating fluid. A plate to be coated is fed to the rolls, generally face-downwards, and the rotation of the lower roll picks up a pre-determinable quantity of fluid which is deposited on the face of the plate, thus coating it. Often this method of coating a sheet is unsatisfactory, and the sheet is then run a second time, to recoat the sheet. It is believed that rerunning the sheet redissolves part of the coating and negates the effect of entrapped air bubbles and dirt particles which may have been incorporated in the first coating.
Another coating machine which purports to eliminate the problems of air bubble and dirt particle entrapment, includes a brush with a back and forth motion, in combination with the drive rolls of the aforementioned prior art machine. In each of the prior art machines the basic problem of slippage of a wet plate between a pair of coupled drive rolls, remains. In operation, attempts to overcome the slippage problem include increasing the pressure on the drive rolls. Increasing the pressure on the drive rolls is not only detrimental to the surface of the rolls, but also makes it more difficult to provide an even coating. Particularly where a diazo coating fluid is utilized, the chemical action of the fluid tends to harden the rolls and complicates the problem of obtaining a uniform coating on the plate. The results obtained with the basic-unit processor are different from those obtained with prior art machines and this difference in results would not be expected by one skilled in the art.
In addition to being useful as a coater for sensitizing a plate, the basic-unit processor of this invention may be used with equal facility as a developer of exposed offset plates, whether the plates are to be developed additively or subtractively, and as a washing apparatus for developed offset plates. Finally, the basic-unit processor may again be used as a coater for sealing or gumming the washed plate. Each of the foregoing functions of the basic-unit processor is discharged with only a change of processing fluid and simple changes in operation necessitated by the process fluid used. Moreover, each function is discharged utilizing a rectilinear reciprocating motion of the rubbing means and a positive sychronous drive for drive rolls in each pair of drive rolls through which the plate is transferred. It is well-known that a rubbing action is essential for coating and developing plates, and prior art devices have variously utilized rotary, orbital and rectilinear rubbing motions. Surprisingly, rectilinear reciprocation is found to be the most advantageous rubbing motion which is equally effective for any basic step in the processing of an offset plate.
From a practical point of view, the more successful machines whether for coating or for developing offset plates are relatively large and enormously complicated. Not unexpectedly, these require a capital investment which precludes their acquisition and operation by a small user of plates. With respect to size, it is noted that available machines, for plates from about 25 inches to about 33 inches in width, will not pass, fully assembled, through a standard 36-inch wide doorway and consequently must be assembled at the installation site, preferably by a qualified factory representative. Similarly, a factory-trained maintenance person is generally required to diagnose and rectify malfunctions of prior art machines because the complicated interrelationship of moving parts and a large number of electrical circuits precludes maintenance by persons normally employed in a printing shop. Thus there has been a growing need for a simple, inexpensive, and safe-to-use automatic plate processor which will not only be easy to acquire and easy to maintain, but which will not be detrimental to the disposal systems conventionally used for sewage treatment. Above all there is a well-established need for a small basic-unit processor which is versatile, and mechanically as ruggedly reliable as its results are, from a printer's viewpoint, routinely reproducible. This invention is directed to such a need.